The demand for continued improvement in the efficiency of combustion turbine and combined cycle power plants has driven the designers of these systems to specify increasingly higher turbine inlet temperatures. Although nickel and cobalt based superalloy materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures sometimes exceeding 1000° C.
It is known in the art to coat a superalloy metal component with an insulating ceramic material to improve its ability to survive high operating temperatures; see, for example, U.S. Pat. No. 4,321,310 (Ulion et al). It is also known in the art to coat the insulating ceramic material with an erosion resistant material to reduce its susceptibility to wear caused by the impact of the particles carried within the hot gas flow path; see, for example, U.S. Pat. Nos. 5,683,825 (Bruce et al.) and U.S. Pat. No. 5,562,998 (Strangman). U.S. patent application Ser. No. 09/393,417, filed on Sep. 10, 1999 taught air plasma sprayed TBC coatings of 50 micrometer to 350 micrometer thickness, applied to superalloy base substrates, for turbine application. There, the TBC coating had a planar grained microstructure, where an overlay was allowed to infiltrate the TBC bulk, completely or partially fill microcrack volumes generally parallel to the superalloy base substrate, and finally react with the TBC material. This was to provide a sintering inhibitor, as well as a coating with a low thermal conductivity, which is also erosion and corrosion resistant.
Much of the development in this field of technology has been driven by the aircraft engine industry, where turbine engines are required to operate at high temperatures and are subjected to frequent temperature transients as the power level of the engine is varied. A combustion turbine engine installed in a land-based power generating plant is also subjected to high operating temperatures and temperature transients, but it may also be required to operate at full power and at its highest temperatures for very long periods of time, such as for days or even weeks at a time. Prior art insulating systems are susceptible to degradation under such conditions at the elevated temperatures demanded in the most modern combustion turbine systems.
In particular, with regard to air plasma sprayed (APS) TBC's, due to repeated thermal cycling, these coatings have to readily accommodate the thermal expansion mismatch stresses and thermal strains to remain adherent to the superalloy substrate. Typical APS coatings achieve this by porosity which is deliberately introduced during the deposition process, such as inter splat boundaries and micro-cracks within the ceramic splats. With increasing demands for higher efficiency of engines, the gas path temperatures are expected to rise and consequently the temperatures at the surface of the ceramic TBC. Higher temperatures would then lead to accelerated sintering of cracks and pores in the APS coatings, especially at the surface. Sintering results in densification of the coating and can lead to its early spallation, due to its reduced capacity to accommodate thermal cycling. Stresses due to thermal cycling can be relieved by vertical cracks through the coating, which increases the thermal cyclic life of the coating.
These vertical cracks in APS coatings can result during the air plasma spraying process, as described in the many articles published in the field of thermal barrier coatings, for example, “Thermal Spray: Advances in Coatings Technology—Experimental and Theoretical Aspects of Thick Thermal Barrier Coatings for Turbine Applications,” G. J. Wilms et al., Proceedings of the National Thermal Spray Conference, Sep. 14-17, 1987, Ed. D. L. Houck pp. 155-166. There, APS spraying at high substrate temperatures was described as inducing vertical segmentation cracks which form while relieving shrinkage stresses within the deposited TBC upon cooling. Initiation of segmentation cracks during APS spraying at high substrate temperatures of the TBC is shown in FIGS. 12 and 13 of the Wilms et al. article, where a brick-like microstructure is shown, and also in FIGS. 3 and 4 where a more monolithic structure is shown, as in FIG. 6. Preferred thick TBC's, over about 2 mm, are described as being dense, less than 15% porosity, but where individual planar platelets are microwelded to each other and connected to their sublayers with a fine network of vertical segmentation cracks, rather than being porous, about 20% or greater porosity.
Coatings deposited by the APS process, with vertical cracks are called segmented TBCs. Formation of vertical cracks in APS coatings are also discussed in U.S. Pat. Nos. 4,457,948; 5,073,433; 5,743,013 and 5,839,586 (Ruckle et al., Taylor, Taylor et al. and Gray et al., respectively), in European Patent 0 705 912 A2, and also in “Crystalline Growth Within Alumina and Zirconia Coatings with Coating Temperature Control During Spraying,” A. Haddadi et al., Thermal Spray: Practical Solutions for Engineering Problems, C. C. Brendt (Ed.), ASM International, Materials Park Ohio, 1996, pp. 615-622; “Taguchi Analysis of Thick Thermal Barrier Coatings,” J. B. Nerz et al., Thermal Spray Research and Applications, Proc. 3rd National Thermal Spray Conference, Long Beach Calif., 1990, pp. 669-673; and “Enhanced Atmospheric Plasma Spraying of Thick TBCs by Improved Process Control and Deposition Efficiency,” E. Lugscheider et al., Proc. 15th International Thermal Spray Conference, 1998, pp. 1583-1588.
J. Wigren et al., in “A Combustor Can with 1.8 mm Thick Plasma Sprayed Thermal Barrier Coatings,” International Gas Turbine and Aeroengine Congress and Exhibition Proceeding, American Society of Mechanical Engineers, 1998, pp. 1-10, taught a series of temperature cycles between 330° C. and 340° C. over time to induce branched segmentation cracks, for thick protective TBC coatings on combustor walls. Such branchings were also described by J. Wigren et al. in “Thermal Barrier Coatings—Why, How, Where and Where To,” Proceedings of the 15th International Thermal Spray Conference, pp. 1531-1542, May 25-29, 1998, where it was pointed out that sophisticated TBC's have raised the temperature capability of gas turbines by about 500° C. in the last 15 years.
While these patents and articles discuss induced microcracks in ceramic coatings, other articles discuss filling such microcracks, primarily to act as seals to corrosive agents, for example, “Effects of Sealing Treatment and Microstructural Grading upon Corrosion Characteristics of Plasma Sprayed Ceramic Coating,” Y. Kimura et al., Proc. 7th National Thermal Spray Conference 1994, pp. 527-536; “Sealing of Plasma Sprayed Ceramic Coatings by Sol-Gel Process,” K. Moriya et al., 7th National Thermal Spray Conference 1994, 549-553; and “Ceramic Impregnation of Plasma Sprayed Thermal Barrier Coatings,” J. Karthikeyan et al., Thermal Spray: Practical Solutions for Engineering Problems, ASM International, 1996, pp. 477-482.
The above-mentioned patents articles, however, do not address the possibility of sintering the vertical cracks and the subsequent lose in strain compliance with increasing operating temperatures.
Accordingly, it is an object of this invention to make a device which is capable of operating at temperatures in excess of 1200° C. for extended periods of time, with reduced component degradation. It is also an object of this invention to provide a method of producing such a device that utilizes only commercially available material processing steps and inexpensive deposition techniques, such as APS, rather than electron beam physical vapor deposition (“EB-PVD”). The APS process basically involves spraying TBC powders, such as stabilized zirconia, after passing them through a plasma gun.